Removes outdated note in documentation. (#4986)

## Description

Trait constraints are already implemented so remove outdated note in
documentation.

## Checklist

- [x] I have linked to any relevant issues.
- [x] I have commented my code, particularly in hard-to-understand
areas.
- [x] I have updated the documentation where relevant (API docs, the
reference, and the Sway book).
- [x] I have added tests that prove my fix is effective or that my
feature works.
- [x] I have added (or requested a maintainer to add) the necessary
`Breaking*` or `New Feature` labels where relevant.
- [x] I have done my best to ensure that my PR adheres to [the Fuel Labs
Code Review
Standards](https://github.com/FuelLabs/rfcs/blob/master/text/code-standards/external-contributors.md).
- [x] I have requested a review from the relevant team or maintainers.

docs/book/src/advanced/generic_types.md

@@ -30,8 +30,6 @@ purely shorthand for the sake of ergonomics.
 
 ## Trait Constraints
 
-> **Note** Trait constraints [have not yet been implemented](https://github.com/FuelLabs/sway/issues/970)
-
 Important background to know before diving into trait constraints is that the `where` clause can be used to specify the required traits for the generic argument. So, when writing something like a `HashMap` you may
 want to specify that the generic argument implements a `Hash` trait.